A sintered R—Fe—B based magnet, including an R2T14B type compound as a main phase, is known as a permanent magnet with the highest performance, and has been used in various types of motors such as a voice coil motor (VCM) for a hard disk drive and a motor for a hybrid car and in numerous types of consumer electronic appliances.
As a sintered R-T-B based magnet loses its coercivity HcJ (which will be simply referred to herein as “HcJ”) at high temperatures, such a magnet will cause an irreversible flux loss. For that reason, when used in a motor, for example, the magnet should maintain HcJ that is high enough even at elevated temperatures to minimize the irreversible flux loss.
It is known that if R in the R2T14B type compound phase is partially replaced with a heavy rare-earth element RH (Dy, Tb), HcJ of a sintered R-T-B based magnet will increase. It is effective to replace a significant percentage of R in the R2T14B type compound phase in the sintered R-T-B based magnet with such a heavy rare-earth element RH to achieve high HcJ even at a high temperature.
However, if the light rare-earth element RL (Nd, Pr), which is R in a sintered R-T-B based magnet, is replaced with the heavy rare-earth element RH, HcJ certainly increases but the remanence Br (which will be simply referred to herein as “Br”) decreases instead, which is a problem. Furthermore, as the heavy rare-earth element RH is one of rare natural resources, its use should be cut down.
Thus, in order to increase HcJ of a sintered R-T-B based magnet, a method for increasing HcJ while minimizing a decrease in Br by supplying a heavy rare-earth element RH such as Dy or Tb onto the surface of a sintered magnet using some evaporation means and then by making that heavy rare-earth element RH diffuse inside of the magnet has been proposed recently.
Patent Document No. 1 discloses a so-called “evaporation diffusion process” in which sintered R-T-B based magnet bodies 1 and RH diffusion sources 2, including a heavy rare-earth element RH, are arranged in a process chamber 11 so as to be spaced from each other as shown in FIG. 7 using a sintered magnet body holding member 3 (which may be an Nb net), a diffusion source holding member 4 and spacer members 12 and are heated to a predetermined temperature. In this manner, the heavy rare-earth element RH can be diffused inside of the sintered R-T-B based magnet bodies 1 while being supplied from the RH diffusion sources 2 onto the surface of the sintered R-T-B based magnet bodies 1.
According to the method disclosed in Patent Document No. 2, a vaporizable metallic material including at least one of Dy and Tb and sintered R-T-B based magnets are housed in a process vessel and are heated to a predetermined temperature within a vacuum atmosphere, thereby vaporizing and depositing the vaporizable metallic material on the sintered R-T-B based magnets and diffusing Dy and Tb atoms of the deposited metallic material over the surface and/or through the crystal grain boundaries of that sintered magnets.
According to Patent Document No. 2, the vaporizable metallic material and the sintered R-T-B based magnets are vertically stacked one upon the other with spacers interposed between them. Each of those spacers is obtained by patterning a wire rod into a grid shape and attaching a supporting member, which is bent substantially perpendicularly upward, to its outer periphery. Using spacers with such a supporting member, the vaporizable metallic material and the sintered R-T-B based magnets are arranged so as to be spaced apart from each other.